In general, a mold for producing forged articles is required to have wear resistance for increasing durability and sliding characteristics with a reduced coefficient of friction for suppressing abrasion due to sliding with the surfaces of forged articles. Therefore, a hard film is formed on the surface thereof. In recent years, for weight reduction of forged articles, application of aluminum materials (containing an aluminum alloy material, the same applies hereinafter) is increased. Since aluminum materials are soft, in hot-forging and warm-forging, they undergo large deformation while being forged, and a newly formed surface is exposed to be in contact with the mold, whereby seizure is likely to occur on the surface of the forged article.
In warm and hot forging, in order to prevent seizure (to impart seizure resistance), a lubricant is deposited on the surface of the mold by spraying and other so that the mold and the forged article are not in direct contact with each other. To this end, a mold whose surface properties are defined to allow retention of the lubricant deposited on the surface thereof has been developed. In Japanese Patent Laid-Open Publication No. 2009-61464, a mold for warm and hot forging which is covered with a hard film comprising a nitride or a carbonitride such as Ti and Cr, and has an arithmetic averaged roughness of the surface Ra defined to be in the range from 0.1 to 0.6 μm is disclosed. Japanese Patent Laid-Open Publication No. 2010-99735 discloses a forging mold having a layer comprising a nitride of a metal mainly composed of Al which is further laminated over a coating a layer comprising boride (TiB2) of Ti, and a surface with an arithmetic averaged roughness Ra defined to 0.05 μm or lower. Japanese Patent Laid-Open Publication No. 2002-307129 discloses a tool for warm and hot processing in which a metal layer such as Ti and Cu is further laminated over a coating layer comprising a nitride or a carbonitride such as Ti and Cr, and the ten point averaged roughness Rz of its surface is defined to be in the range from 4 to 15 μm.
However, definition of the height (depth) and height difference of the unevenness on the surface of the mold such as Ra and Rz only is insufficient for retaining the lubricant. In particular, in plastic processing of a soft metal such as an aluminum material, seizure may occur under a high pressure of the contacted surface during forging.